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Gmane
From: Alistair Crooks <agc-Q8AzPd93frYdnm+yROfE0A <at> public.gmane.org>
Subject: Re: upstream source code authenticity checking
Newsgroups: gmane.comp.security.oss.general
Date: Thursday 25th April 2013 05:55:59 UTC (over 4 years ago)
On Wed, Apr 24, 2013 at 10:19:15PM -0400, Eric H. Christensen wrote:
> This is a good discussion to have.  I've recently started working on
> "best practices" articles at Red Hat and feel this would make an
> excellent article on how we can all improve the security of our
> source code that inevitably gets pushed into the various
> distributions. 
> 
> What is really the best, most proper way of desiminating releases? 
> I really don't like the use of MD5 for checksums (I'd prefer
> something out of the SHA-2 or SHA-3 family of hashing algorithms)
> and I really *do* like the use of PGP for signing the code.  I do
> foresee some practices within the use of PGP that might not be
> great, though.
> 
> So what is the best way of authenticating the source code?

My apologies - obviously I didn't make myself clear previously.

Verifying a signature on a (SHA1, by default) digest will get you the
information that a person with access to the private key said that the
digest was calculated at a certain time.  It also leads to the
following questions:

Q1. who has access to the private key?

A1.  divulgence of creds and the key itself may be known or not, so
even with the most stringent key protection and hygiene in place, the
answer to (1) is "unknown"

Q2. is the key protected by a passphrase?

A2. We'll never know. It's irrelevant too, due to Q1/A1

Q3. has the key expired or been revoked?

A3. At last, one we can answer. Of course we'll know when it expires.
Unfortunately, revocation may make itself to key servers, maybe not.
And even if it did, what was the last time of refresh for the public
key that we have?

Q4. where's the public key for this?

A4. could be anywhere. If it's on one of the HKP servers, then cool.
Not, however, that it can be verified - I know of at least one person
who has had pubkey information uploaded to the key servers for a key
he had no knowledge about. Anyone can put whatever email address into
the userid that they want. If it came with the tarball, ho hum.

Q5. what was signed?

A5.  if it comes out as a text document, according to RFC 4880, it has
some weird properties; hopefully all tar files will be binary. 
Whatever, what was signed was something with the same digest as the
tarball.  Default algorithm is SHA1.  Second pre-image attacks on SHA1
are getting closer to being possible, and there are means to modify
entries in the tarball so that an attack is much easier.

Q6. Is this a DSA key?  (DSA keys rely on good entropy at signing
time) If so, how good was the entropy on the machine used to generate
the signature?

A6.  Again, unknown.

Q7. Has someone found the k value for Q6/A6 previously?

A7. They might have done. We'd only know if they told us.

Q8. Did anyone have root access to the machine where gpgagent was
running?

A8. unknown

So, all in all, what you have is a digest, signed by someone who knows
the key, or who has access to the creds (if any) for the key, or who
has found out the key creds, albeit with timestamp info for when the
signature took place.

I'm not sure what using PGP gains us?

Regards,
Alistair
 
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